1. Technical Field
The present invention relates to audio systems, and more particularly to motional feedback related control of loudspeakers in an audio system.
2. Related Art
Audio systems typically include an audio source providing audio content in the form of an audio signal, an amplifier to amplify the audio signal, and one or more loudspeakers to convert the amplified audio signal to sound waves. Loudspeakers are typically indicated by a loudspeaker manufacturer as having a nominal impedance value, such as 4 ohms or 8 ohms. In reality, the impedance of a loudspeaker, and thus the load on an amplifier driving the loudspeaker varies with frequency. In addition the impedance of a loudspeaker is often significantly non-linear. Variations in loudspeaker impedance with respect to frequency may be shown with a loudspeaker impedance curve, which is often provided by the manufacturer with a manufactured model of a loudspeaker. The non-linear aspects of the load impedance are never published.
Non-linear operation of a loudspeaker is also due to it being an electro-mechanical device that is sensitive to variations in voltage and current. Further, non-linear operation of a loudspeaker may be due to displacement and aging, as well as environmental conditions, such as temperature and humidity. In addition, during operation a loudspeaker voice coil may be subject to heating and cooling dependent on the level of amplification of the audio content. Moreover, variations in manufacturing and materials among a particular loudspeaker design may also cause/contribute to non-linear operation of a loudspeaker. As such, loudspeaker parameters such as the DC resistance, moving mass, compliance, resonance frequency and inductance may vary significantly among the same manufactured model of a loudspeaker, and also may change significantly as operating and environmental conditions change.
Since non-linear operation of the loudspeaker and the corresponding changing impedance of the loudspeaker can vary dramatically for any given loudspeaker, resulting distortion when the loudspeaker is driven by an amplifier with an amplified audio signal can also be dramatically different among different loudspeakers, and under different operating to conditions.